1. Field of the Invention
The present invention relates to the field of semiconductor packaging and more particularly to a bare die package and its method of fabrication.
2. Discussion of Related Art
Flip chip technology is commonly used in the semiconductor industry for interconnecting a processed die to a package substrate. FIG. 1 illustrates a typical bare die package. The bare die package comprises a processed die 110 electrically coupled to a package substrate 120. Die 110 and package substrate 120 are typically made from different materials, and hence both have different coefficients of thermal expansion (CTE). At the package assembly process, the die 110 does not expand as much as the package substrate 120, thus causing the package substrate 120 to bend away from the die 110 as shown in FIG. 1.
The bare die package can be attached to a socket or printed circuit board (PCB) by various methods such as Pin Grid Array (PGA), Ball Grid Array (BGA) or Land Grid Array (LGA). In LGA compression mount technology (CMT), the bare die package is placed on a socket before a heat sink is disposed on top of the bare die package. FIG. 2 illustrates the different types of force applied onto the package substrate 120 if the bare die package is secured between the socket and heat sink (not shown). Loading force 181 refers to the force applied onto the processed die 110 by the heat sink, and loading force 182 refers to the force applied onto the package substrate 120 by the socket. As a result, the ends of the package substrate 120 bend upward towards the die 110, thereby causing excessive warpage of the package substrate 120 or commonly referred to as substrate warpage.
In one area of LGA technology, an integrated heat spreader (IHS) is formed on top of the die. The IHS not only dissipates some of the heat from the die but also serves as a protective lid for the die. FIG. 3 illustrates a conventional LGA package comprising a IHS 130 attached to the die 110 that is coupled to the package substrate 120. The IHS 130 includes a top plate 131 having sidewalls 132, wherein the sidewalls 132 are attached to the package substrate 120 by a sealant 150. Top plate 131 is coupled to a thermal interface material (TIM) 160 attached to the top of the die 110. As shown in FIG. 3, the IHS 130 provides an additional structural support to the package substrate 120, and also minimizes the effects of substrate warpage.
However, a bare die package does not utilize a IHS. Without a IHS, the bare die package may experience excessive substrate warpage when it is secured between a socket and heat sink. Warpage of the package substrate may result in improper electrical contact between the package substrate and the socket. More significantly, substrate warpage can damage and affect the functionality of the die. One way of minimizing substrate warpage is by placing a compressible gasket between the package substrate and the heat sink, where the compressible gasket surrounds the die. When a heat sink is mounted onto the die, the gasket provides some structural support to the package substrate and minimizes warpage. However, the package substrate is still prone to warpage because the compressible gasket does not provide sufficient structural support to the package substrate.